Dual-voltage vehicle electrical systems are known, for instance, from motor vehicle technology, especially for hybrid or electric vehicles. Because of the increasing number of electrical users, a single voltage supply is often no longer sufficient. Furthermore, electrical users are being applied which require a higher operating voltage, so that dual-voltage vehicle electrical systems have been developed which have different voltage levels. It is customary, in this context, to connect these vehicle electrical systems via a direct voltage converter, frequently also designated as a DC voltage transformer, so as to supply these two vehicle electrical systems by a common generator.
Within the scope of an external start, or even because of a phase inversion caused in a different manner, a polarity reversal may take place on the low-voltage side of the DC voltage transformer. To be sure, DC voltage transformers may be designed to be protected against polarity reversal, up to a certain current level and limited as to time, but to avoid long-term damage to the DC voltage transformer, it is necessary reliably to detect a polarity reversal on the low-voltage side of the DC voltage transformer, and to convert the dual-voltage vehicle electrical system to a reliable operating state, if necessary.
A control unit protection for a dual-voltage vehicle electrical system is discussed, for example, in German document DE 199 61 622 A1, including two vehicle electrical systems of different voltage levels galvanically connected to each other, in which electrical users are situated, in which a protective diode is connected in the non-conducting direction in parallel to the electrical users in the vehicle electrical system having the lower voltage.